Torque represents rotational force, the key element in rotating applications such as those that make use of ball bearing assemblies. There are a number of different varieties of torque as well as a variety of factors that affect torque. The following addresses torque as it affects the motion of a ball bearing.
There are two basic kinds of torque: Starting torque and running torque.
Types of Bearing Torque: Starting Torque
Starting bearing torque comprises of two main elements which need to be overcome before rotation occurs:
(1)Metal to Metal contact between Raceways and Balls
Both the balls and the races of the ball bearing are typically made of stainless steel, either entirely or in part. There is friction that naturally occurs between two pieces of metal, which is why lubricant is required for smooth operation. In addition, metal contacting metal can cause wear and deformation on both parts, which makes proper lubrication absolutely crucial. However, even with proper lubrication, bearing torque is required to overcome the friction generated by these two metal elements coming in contact with one another.
(2) Lubricant shearing
The force applied through the use of the bearing shears the lubricant so that it has a lower viscosity for smooth, continuous operation of the bearing. However, sufficient bearing torque is required to shear the lubricant so that it reaches this viscosity point.
Types of Bearing Torque: Running Torque
While starting torque is the torque required to get a bearing started, running torque is the torque required to keep a bearing running.
Running torque is comprised of retainer drag (on both rings and balls) and lubricant churning caused by couplings between balls and retainer, and retainer and raceways. The amount of retainer drag will depend on the type of retainer and retainer materials you are using. You will get more drag from a seal than a shield, since the seal is in contact with both rings of the bearing while the shield is not. However, the seal provides greater protection against contamination. Lubricant churning is the loss of viscosity in your lubricant due to continued agitation of the lubricant because of the action of the bearing.
Factors Related to Torque for a Ball Bearing
Torque has a direct effect upon the following: temperature generation, speed variation, power consumption (at start-up and during running), and also power consumption variations caused by unstable rotation, etc.
The primary cause of temperature generation in ball bearings is due to churning from several lubricant couplings. Churning type lubricants cause temperature generation because of the shearing effect. Countermeasures for this problem are to reduce the lubricant quantity or to change to a channeling type of lubricant.
Failure to Reach Speed
Sometimes motors cannot reach the designed nominal speed because of problems caused by:
Excessive Power Consumption
The reasons for this are the same as "Failure to Reach Speed" above. Lubricants with high Worked Penetration can also add to the problem.
Excessive Reach Running Current
The high running current of the motors during operation is considered due to:
This is caused by grease "slumping" into the pathway of the ball. The solutions to this problem are:
Relationship Between Speed and Running Torque
Torque increases with speed.
Relationship Between Grease Quantity and Running Torque
Generally, if the lubricant quantity is increased, the running torque is increased.
Relationship Between Temperature and Running Torque
Generally, as temperature decreases, Bearing Torque increases.
More on Bearing Lubrication and Torque
As should be apparent, the type and amount of lubrication used can have a significant effect on torque for a bearing. Excessive bearing lubricant can increase running torque and can also result in speed fluctuations. Lubricant can also affect power consumption and temperature fluctuations. The following section will explore bearing lubrication in more depth, but when trying to understand the issue of torque with regard to your bearing, it will also be helpful to understand bearing lubrication.
Lubrication for a bearing generally means grease, which is oil with a thickening substance added. You can use just oil for some bearing applications that require very low torque. Thickening agents for bearing grease can include lithium, urea, and PTFE (polytetrafluoroethylene). Grease is a highly effective bearing lubricant for a number of reasons. For one, it enables lubricant shearing, which can aid in continuous operation for the bearing. For another, grease can keep an application lubricated for an extended period of time, which is important, as a ball bearing is sealed and not re-lubricated. Grease can also act as a sealant to protect against bearing contaminants. Grease is also relatively inexpensive.
Urea grease tends to be used over lithium grease for those bearing-based applications in which heat and water resistance are a priority, as urea has been shown to be a stronger bearing grease in these areas. PTFE grease is also becoming increasingly popular as a highly durable grease that tends to last a long time.